qemu_command: Resolve Coverity DEADCODE

qemu_command: Resolve Coverity DEADCODE

One useless warning, but the other one rather pertinent. On entry
the 'trans' variable is initialized to VIR_DOMAIN_DISK_TRANS_DEFAULT.
When the "trans" was found in the parsing loop it def->geometry.trans
was assigned to the return from virDomainDiskGeometryTransTypeFromString
and then 'trans' was used to do the comparison to see if it was valid.

So remove 'trans' and use def->geometry.trans properly